1. Field of the Invention
The present invention relates to a chemical vapor deposition (referred to as CVD in this specification) system, and more particularly to a CVD system suited for forming a film on a large flat panel substrate.
2. Description of the Related Art
As a manufacturing method of liquid crystal display, a method of using high temperature polysilicon TFT (thin film transistor) and a method of using low temperature polysilicon TFT have been known. In the manufacturing method of using high temperature polysilicon TFT, in order to obtain a silicon oxide film of high quality, a quartz substrate which can be fit for a high temperature exceeding 1000xc2x0 C. is used. By contrast, in manufacture of low temperature polysilicon TFT, an ordinary glass substrate for TFT is used, so that it is necessary to form a film at low temperature (for example, 400xc2x0 C.). The manufacturing method of liquid crystal display by using low temperature polysilicon TFT does not require any special substrate, and is hence widely employed recently, and its production is expanding.
In manufacture of liquid crystal display by using low temperature polysilicon TFT, when forming a silicon oxide film appropriate as gate insulating film at low temperature, plasma enhanced CVD is used.
When forming a silicon oxide film by the plasma enhanced CVD, silane and tetraethoxy silane (TEOS) are used as representative material gas.
When forming a silicon oxide film by plasma enhanced CVD, using silane or the like as material gas, in a conventional plasma enhanced CVD system, the material gas and oxygen are introduced in the front space of the substrate, plasma is produced by mixed gas of material gas and oxygen, and the substrate is exposed to the plasma, so that a silicon oxide film is formed on the surface of the substrate. In such a conventional plasma enhanced CVD system, the material gas is directly supplied into the plasma produced in the plasma enhanced CVD system. Accordingly, in the conventional plasma enhanced CVD system, ions of high energy are injected from the plasma existing in the front space of the substrate to the film forming surface of the substrate, and the silicon oxide film is damaged, and film properties are impaired. Further, since the material gas is directly introduced into the plasma, the material gas and plasma react violently with each other to generate particles, thereby lowering the yield.
To solve the problems, in the previous Japanese Patent Application (unexamined Japanese Patent Publication No. JP P2000-345349A), it has been attempted to improve the conventional plasma enhanced CVD system, and a new CVD system was proposed.
The CVD system proposed in JP P2000-345349A is a system for producing plasma in a vacuum container to generate radicals, and forming a film on the substrate by the radicals and material gas. A conductive partition wall is disposed in the inside of the vacuum container. Thereby, the inside of the vacuum container is separated by the conductive partition wall into two compartments. One of these two compartments is formed as a plasma generating space containing high frequency electrode, and the other is formed as a film forming space with a substrate holding mechanism for mounting substrate. The conductive partition wall has plural penetration holes for communicating between the plasma generating space and film forming space, and also has an inner space separated from the plasma generating space and communicating with the film forming space through plural diffusion holes. The material gas is supplied from outside into the inner space of the conductive partition wall, and is introduced into the film forming space through the plural diffusion holes. On the other hand, radicals formed in the plasma generating space are introduced into the film forming space through the plural penetration holes opened in the conductive partition wall. Herein, the size (length and diameter) and structure of the penetration holes and diffusion holes are determined so that the material gas introduced in the film forming space may not diffuse reversely into the plasma generating space as for the penetration holes, and so that the radicals introduced in the film forming space may not diffuse reversely into the inner space of the conductive partition wall as for the diffusion holes.
By the CVD system proposed in JP P2000-345349A, worsening of film properties of silicon oxide film formed on the glass substrate can be prevented, and the product yield can be improved.
However, by the CVD system proposed in JP P2000-345349A, when a silicon oxide film is formed on a glass substrate of a wide area, for example, 370 mmxc3x97470 mm, it was often insufficient in the aspect of uniformity, in both film thickness and quality of silicon oxide film. That is, in a conductive partition wall 14 of the CVD system proposed in JP P2000-345349A shown in an enlarged view in FIG. 5, a local abnormal discharge (hollow cathode discharge) is induced in an inner space 32 of the opening at the side of a plasma generating space 15 of penetration holes 25 communicating between the plasma generating space 15 side and film forming space 16 side divided by the conductive partition wall 14, which has been considered to lead to unstable plasma.
The penetration holes of the conductive partition wall which determine the uniformity of film quality and thickness of the silicon oxide film formed on the wide substrate are most important parts demanding the highest precision in manufacture because they have essential functions for preventing gas leak from the inner space of the conductive partition wall filled with material gas, preventing abnormal discharge in the penetration holes, and realizing smooth transfer of neutral radicals
Therefore, the CVD system proposed in JP P2000-345349A had a room for further studies in the aspects of performance such as improvement of uniformity of thickness and quality of the film formed on the substrate, optimum structure for penetration holes, and means for manufacture.
It is hence an object of the present invention to present a CVD system capable of forming a film of uniform thickness and uniform quality over a wide area, by enhancing the plasma stability, operating stably and continuously, achieving a high product yield, by further improving the CVD system proposed in JP P2000-345349A capable of preventing inverse diffusion of material gas into the plasma forming region, in the case of forming a silicon oxide film on a wide substrate by using material gas such as silane, on the basis of the CVD using radicals generated by plasma, in manufacture of large liquid crystal display using low temperature polysilicon TFT.
It is also an object to present a CVD system capable of stably maintaining many functions of the conductive partition wall, especially the penetration holes, including prevention of gas leak from the conductive partition wall filled with material gas, prevention of abnormal discharge in the penetration holes communicating from the plasma generating space to the film forming space, and efficient transfer of neutral radicals from the plasma generating space side to the film forming side.
To achieve these objects, the CVD system of the present invention is characterized by the following structure.
That is, the CVD system of the present invention is a CVD system for producing plasma in a vacuum container to generate radicals, and forming a film on the substrate by the radicals And material gas. A conductive partition wall is disposed in the inside of the vacuum container. Thereby the inside of the vacuum container is separated by the conductive partition wall into two compartments. One of these two compartments is formed as a plasma generating space containing high frequency electrode, and the other is formed as a film forming space with a substrate holding mechanism for mounting substrate. The conductive partition wall has plural penetration holes for communicating between the plasma generating space and film forming space, and also has an inner space separated from the plasma generating space and communicating with the film forming space through plural diffusion holes. The material gas is supplied from outside into the inner space of the conductive partition wall, and is introduced into the film forming space through the plural diffusion holes. A high frequency electric power is applied to the high frequency electrode to generate plasma discharge in the plasma generating space. And radicals formed in the plasma generating space are introduced into the film forming space through the plural penetration holes opened in the conductive partition wall.
In this CVD system, plasma is generated by using oxygen gas, and a thin film is deposited on the surface of a substrate by using material gas such as silane. And the inner space of the vacuum container used as the treating compartment is separated by a conductive partition wall into a plasma generating space and a film forming space. So that the processing surface of the substrate disposed in the film forming space is not exposed to the plasma. Besides, being separated by the conductive partition wall, the material gas introduced in the film forming space is sufficiently prevented from moving to the plasma generating space side. That is, the conductive partition wall has plural penetration holes, and the plasma generating space and film forming space at both sides of the conductive partition wall communicate with each other only through the penetration holes, and the size and structure of the penetration holes are determined so that the material gas introduced in the film forming space may not diffuse reversely into the plasma generating space side.
The size and structure of the penetration holes are same as proposed in the previous Japanese Patent Application (JP P2000-345349A), that is, the condition of uL/D greater than 1 is satisfied, where u is the gas flow velocity in penetration holes, L is the substantial length of penetration holes (see FIG. 3, FIG. 4, in these cases, L is the length of the portion of the minimum diameter), and D is the binary diffusivity (mutual gas diffusion coefficient of two types of gases of material gas and process gas, in this case; oxygen gas). In the diffusion holes, too, when the same condition as in the penetration holes is applied, it is effective to prevent the radicals introduced in the film forming space from diffusing reversely into the inner space of the conductive partition wall, and the penetration holes and diffusion holes of the partition wall in the CVD system of the present invention arc formed to satisfy this condition.
It is a feature of the CVD system of the present invention that the diameter of penetration hole at the film forming space side are designed to be equal to or larger than the diameter of penetration hole at the plasma generating space side.
The shape of penetration holes that has an equal or larger diameter at the film forming space side as compared with diameter at the plasma generating space side is realized by, for example, a cylindrical shape from the plasma generating space side toward the film forming space side, or a shape consisting of a cylindrical portion from the plasma generating space side toward the film forming space side and a conical portion widening in diameter consecutive to the cylindrical portion, or a shape consisting of a cylindrical portion from the plasma generating side toward the film forming side and a conical portion widening in diameter consecutive to the cylindrical portion and a cylindrical portion of widen diameter consecutive to the conical portion.
By forming the penetration holes in such characteristic shape, the portion satisfying the hollow cathode discharge condition can be eliminated. As a result, the stability of plasma is enhanced, and abnormal discharge is prevented at the plasma generating space side of penetration holes communicating from the plasma generating space side to the film forming space side, and as a result neutral radicals can be transferred efficiently from the plasma generating space side to the film forming side. Thus, stable operation is realized continuously, which results in a high production throughput, while maintaining the capability of forming a uniform film in thickness and quality over a wide area can be presented.
For example, by the CVD system of the present invention, a film thickness distribution of xc2x15,2%, which is better than in prior plasma enhanced CVD where xc2x110%-15% uniformity is typically obtained, could be obtained on a glass substrate of 370 mmxc3x97470 mm (thickness of silicon oxide film: 200 nm).
In the CVD system of the present invention as the before described, the penetration holes can be formed by a structure independent from the conductive partition wall. Thus, the penetration holes can be processed precisely and at a low cost.